“Now Esau saw that Isaac had blessed Jacob and sent him away to Paddan-aram to take a wife from there, and that as he blessed him he charged him, ‘You shall not marry one of the Canaanite women.’ Jacob had obeyed his father and his mother and gone to Paddan-aram. So when Esau saw that the Canaanite women did not please Isaac his father, Esau went to Ishmael and took Mahalath the daughter of Abraham’s son Ishmael, sister of Nebaioth to be his wife in addition to the wives he had.”
Esau saw that Isaac had blessed Jacob and sent him away to find a wife that was not a Canaanite. Esau then took Mahalath as his wife, in addition to his other two wives. She was Abraham’s half brother Ishmael’s daughter, his first cousin, and the sister of Nebaioth of Edom. Thus he was trying to keep his marriage within the family, only it is with Ishmael’s family.